In manufacturing monocrystalline silicon semiconductor devices it is common practice to begin with a slice of monocrytalline silicon referred to as a wafer and to fabricate one or more devices from the wafer. In a wide variety of differing forms the monocrystalline silicon elements of the devices produced exhibit laterally sloping side walls. The typical way of forming such laterally sloping side walls is by etching.
Typically etching is performed by masking selected areas so that an etchant is free to contact the monocrystalline silicon substrate in the remaining areas. When isotropic etching is performed, the silicon substrate is removed in a directionally nonselective manner. The limitations of isotropic etching can be illustrated by reference to FIG. 1. A monocrystalline silicon substrate 100 is shown provided with parallel, opposed upper and lower major surfaces 101 and 103. To permit etching a conventional masking layer 105 providing an opening 107 is formed on the upper major surface. An isotropic etchant reaching the silicon substrate through the opening removes silicon at an approximately equal rate in all directions. This forms the channel 109 shown.
Isotropic etching produces a number of disadvantages. First, the width of the etch channel is typically wider than the width of the opening in the masking layer. Slight variances in the duration of etching can result in variances in the width of the etch channel. Second, isotropic etching undercuts the masking layer. Undercutting poses an undesirable feature for many subsequent stages of device manufacture. Third, despite being sloped the lateral walls of the etch channel still intersect the upper major surface of the substrate at a high angle, approaching 90 degrees. The high angle of intersection is recognized to present a point of potential weakness where, following masking layer removal, continuous layers are intended to bridge the upper major surface and the channel surface.
The problem of undercutting has been minimized if not obviated by developing anisotropic etching techniques. FIG. 2 illustrates an ideal anisotropic etch. A monocrystalline silicon substrate 200 is shown provided with parallel, opposed upper and lower major surfaces 201 and 203. To permit etching a conventional masking layer 205 providing an opening 207 is formed on the upper major surface. An anisotropic etchant reaching the silicon substrate through the opening removes silicon unidirectionally so that the lateral walls 209 of the etch channel are aligned with the edges of the masking layer opening.
Anisotropic etching avoids or at least minimizes the undercutting problem of isotropic etching. It is not a useful etching approach for forming sloped lateral walls for a semiconductive substrate. Further, the angle of intersection between the upper major surface of the substrate and the etch channel remains undesirably high.
Sugishima et al U.S. Pat. No. 4,352,734 summarizes a variety of conventional isotropic and anisotropic etch techniques for monocrystalline silicon substrates.
Kinoshita et al, "Anisotropic Etching of Silicon by Gas Plasma", Japan J. Appl. Phys., Vol. 16, 1977, No. 2, pp. 381 and 382, reports a form of etching which differs from both conventional isotropic and anisotropic etching. By using carbon tetrachloride to form a gaseous plasma a sloping surface intersecting the major surface of a monocrystalline silicon substrate at an angle of 45 to 50 degrees was formed. Since the silicon substrate major surface lay in a {100} crystallographic plane, it was speculated that the sloped surface formed by etching lay in a {111} crystallographic plane, despite the well established fact that {111} and {100} crystallographic planes intersect at an angle of 54.74.degree.. Experiments with halofluorocarbon and fluorocarbon gases failed to show any etch rate perference as a function of crystallographic direction.